The role of the iroquois homeobox 4A (IRX4) gene has been reported in regulating chamber-specific expression of myosin isoforms and ventricular differentiation in animal models, and potential causal effects of its mutation in a ventricular septal defect in humans have been proposed (Bao et al., 1999; Bruneau et al., 2001; Cheng et al., 2011). Here, IRX4 is linked to ventricular septal defect 1.